According to many here it is too weak for even the 273. My 68 Darts rear end just failed and it's a 273 with auto and I never beat on the car even once. It just failed being driven normally down the street to work. I'm sure many of the rears did hold up though because many of these cars have 200k miles on them.
Sometimes mechanical things just fail. Did you put all 200k miles on your 68 Dart? If so and there was no one event you recall that would have severely stressed it over the last 57 years then chalk it up to metal flaw.